1. Field of the Invention
The present invention relates to a circuit that reads the data about fuse elements. More particularly, the invention relates to a circuit that reads the data programmed in the fuse elements that are provided in, for example, the redundancy circuit of a semiconductor memory device.
2. Description of the Related Art
Two methods are known, which read the data programmed in the fuse elements provided in a semiconductor integrated circuit device. (The data represents whether the fuse elements are conductive or non-conductive.)
The first method is to detect a signal voltage that depends on whether a target fuse element is conductive or non-conductive. The first method is unreliable, however, because the semiconductor integrated circuit device has been manufactured in conditions different from those in which any other circuit has been made, the fuse elements have been cut in different states, the resistance of each non-conductive fuse element decreases with time, the resistance of each conductive fuse element increases with time, and so forth.
The second method is to detect the difference in two signal voltages that depend on a pair of fuse elements, one of which is the target fuse element. This method is disclosed in, for example, the specification of U.S. Pat. No. 6,201,432. Claim 9 in this U.S. patent describes a redundancy-address generating circuit that has a differential amplifier. The differential amplifier detects a difference between two signal voltages output from a pair of circuits, each comprising one fuse element and a resistor element connected in series to the fuse element. In the redundancy-address generating circuit, both fuse elements of the pair may be cut. Since two resistor elements are connected in series to the fuse elements, respectively, the circuit occupies a large area. This makes it impossible to determine how the resistance of each fuses element changes with time.
The specification of U.S. Pat. No. 6,384,666 discloses a system that can vary the threshold voltage of a latch circuit to 16 values in order to cancel out the resistance change that a fuse element undergoes as time passes. This system needs to have a control circuit for varying the threshold voltage. Its circuit configuration is inevitably complex. Consequently, it takes a long time to determine how the resistance of the fuse element changes with time.